Single crystal ingots of commercial interest are generally grown by one of the following processes: (a) Vertical Gradient Freeze (VGF), (b) Vertical Bridgeman (VB), (c) Horizontal Bridgeman (HB), and (d) Liquid Encapsulated Czochralski (LEC). Wafers produced by VGF, VB, and HB can produce "Low Defect Density" (LDD) material; and LEC by its nature produces only "High Defect Density" (HDD) material. By definition, LDD material is characterized herein by etch-pit densities (EPD) in the order of 10.sup.2 to 10.sup.3 dislocations/cm.sup.3 ; and HDD material is characterized by higher EPD in the order of 10.sup.4 to 10.sup.5 dislocations/cm.sup.3. The residual internal stress for crystals grown by LEC is substantially higher than in wafers produced by VGF, VB, and HB. The higher quality of the crystal and the lower residual internal stress of LDD material translates directly to advantages of less cracking when slicing wafers from an ingot, less breakage during wafer handling, better surface morphology, and a number of other properties which are desirable in producing end products, e.g., lasers, semiconductor circuits, etc.
Commercial suitability of a semi-insulating wafer is judged on the basis of controlled resistivity, uniform resistivity across a wafer, mobility, purity, EPD, flatness of wafers, etc. Commercial suitability of semi-insulating ingots for deriving such wafers is judged on the basis of its electrical properties, homogeneity from head to tail of the ingot, low residual internal stresses, and reproducible growth processes.
As seen below herein, controlled incorporation of carbon is a key factor in successful growth of semi-insulating GaAs materials. The semi-insulating electrical properties of GaAs are determined by the concentrations of: (a) residual donor impurities, e.g., Silicon; (b) acceptor impurities which comprise residual carbon in the poly-crystal material and carbon introduced as an acceptor impurity; and (c) EL2 which is a mid-gap intrinsic double donor defect which is related to the stoichiometry of GaAs material. An EL2 defect is associated with an As-on-Ga anti-site. Semi-insulating GaAs can only be achieved when the following relation is established: N(EL2)&gt;[N(a)-N(d)]&gt;0, where N(EL2) is the concentration of EL2 defects, N(a) is the acceptor concentration, principally carbon, and N(d) is the residual donor concentration, e.g., silicon,
EL2 defect density and impurity control, especially the control of acceptors, are critical to achieving suitable semi-insulating properties in GaAs. Carbon is the main and most desirable acceptor; however, controlled incorporation of carbon has proved to be difficult.
In LEC, carbon mainly comes from two sources: (1) carbon is present as an impurity in the raw materials, and (2) carbon contamination comes from the hot graphite furnace components during growth. Semi-insulating GaAs with resistivity greater 10.sup.7 .OMEGA.-cm can generally be produced by LEC without significant difficulty. To achieve control of carbon in LEC, different carbon sources have been investigated, such as barium carbonate and carbon monoxide. So far, using CO is almost the standard in achieving carbon control in LEC. However, controlling the semi-insulating property of resistivity at less than 10.sup.7 .OMEGA.-cm is very difficult in LEC, since LEC has much higher background carbon level than VGF.
In HB, due to the high level of Si contamination from quartz, it is almost impossible to achieve semi-insulating GaAs by satisfying the condition of N(EL2)&gt;[N(a)-N(d)]&gt;0. However, semi-insulating HB materials can be achieved by intentional doping with Chromium.
In VGF and VB, carbon level is normally low in the crystals grown, i.e., approximately low 10.sup.14 /cm.sup.3. Incorporation of carbon in VGF, and in VB is much more difficult than in LEC; and incorporation of carbon therein by use of CO is not practical. Incorporation of Carbon into the crystal is difficult because of the low solubility of Carbon in GaAs. Although the condition of N(EL2)&gt;[N(a)-N(d)]&gt;0 can be satisfied by carefully minimizing the residual donor concentration, only limited semi-insulating properties of GaAs can be achieved. The resulting low level of balance between EL2, the donor impurities and acceptor impurities in such material is not desired since the materials tend to be unstable.
Although it has been possible to achieve limited semi-insulating properties in as grown VB and VGF GaAs ingots, there are no standard ways of controlled incorporation of carbon into GaAs ingots, particularly to achieve high carbon concentrations, e.g., greater than 10.sup.16 atoms/cm.sup.3 and higher.
In the prior art there are many references to carbon doping of GaAs ingots during growth by a use of a variety of carbon compounds, e.g., barium carbonate or carbon monoxide. A paper entitled Low-dislocation-density and Low-residual-strain Semi-insulating GaAs as Grown by Vertical Boat Method by T. Kawase, et. al. reports achieving good control of carbon incorporation in GaAs ingots by use of a "carbon source" in a pBN crucible charged with GaAs poly-crystal material. Since the "carbon source" is not identified in that paper, it is unknown how the reported results are achieved.